An evaluation of the presence or absence of tumor metastasis or invasion has been a significant issue for achieving an effective treatment for cancer patients. The involvement of the lymph system in tumor metastasis is well established. Lymphatic systems are present as widely dispersed tissues, fluids, and cells concerned in a variety of interrelated functions of the mammalian body including the circulation and modification of tissue fluid formed in the capillary beds, and the removal by mononuclear phagocytes of cell debris and foreign matter. The lymphatic system is importantly involved in participation with the blood vascular system in developing the immune response of the lymphocytes and other cells. Lymph flows within the system as a consequence of a variety of perceived mechanisms of organ and tissue dynamics. For certain cancers, metastasis occurring in consequence of lymph drainage will result in an initial location or positioning of neoplastic cells at certain lymph nodes typically deemed "regional nodes" within a pertinent lymph drainage basin. Some cancers, such as those encountered in the breast, will evidence somewhat predictable nodal involvement.
In designing forms of cancer disease management, efforts are directed to the identification of affected lymph nodes. For cancers such as breast cancer, the sites of lymph node involvement are commonly encountered at axillary, internal mammary, and supraclavicular lymph node regions. Of these, the axillary lymph node region is the principal site of regional metastasis from carcinoma of the breast, and approximately 40% of patients have evidence of spread to the axillary nodes. In early approaches to the disease, these axillary nodes were removed as a form of therapy. Presently, however, their positive involvement, or lack thereof, has become the subject of diagnostics as opposed to therapy. In this regard, the combination of the presence and extent of metastasis to the axilla represents the single most important prognostic factor for the management of patients with breast cancer, See generally, "Cancer, Principles and Practice of Oncology", vol. 1, 4th ed. DeVita, Jr., et al., chapter 40, Harris, et al., J. P. Lippincott Co., Philadelphia, Pa. 1993.
The axilla is a triangular region bounded by the axillary vein superiorly, the latissimus dorsi laterally, and the serratus anterior medially, With some diagnostic procedures, essentially all axillary nodes at the axilla assumed to represent the drainage basin are removed during surgery for analysis. In general, somewhere between 10 and 30 nodes will be removed in the course of dissection with, of course, the attendant risks. In this regard, these nodes are generally surrounded by investment of fatty tissue and visualization of them necessarily is limited. Such dissection will pose risks of cutting the long thoracic nerve, the thoracic-dorsal nerve, the nerve to the pectoralis major, or the axillary vein. Morbidity may occur in some cases due to regional node removal and patients are known to frequently discuss a numbing of the arm region following the procedure.
While this form of axillary lymph node dissection has been the conventional approach to determining nodal metastatic involvement, more recent data suggest that less radical axillary node evaluation procedures may generate equivalent information for staging and patient management, but with far more limited dissection and resultant trauma, as discussed below. A procedure to moderate the otherwise somewhat radical axillary lymph node dissection common in staging breast cancer has been described generally in "Lymphatic Mapping and Sentinel Lymphadenectomy for Breast Cancer" by Guiliano, A. E.; Kirgan, B. M.; Guenther, J. M.; and Morton, D. L., Annals of Surgery, vol 220, no. 3: 391-401, 1994. With the procedure, in general, a vital blue dye is injected into the breast mass and surrounding breast parenchyma. Following a relatively short interval, a transverse incision is made just below the hair-bearing region of the axilla. Blunt dissection is performed until a lymphatic tract or duct leading to a blue stained node is identified. The lymph duct, having a blue color, provides a guide path leading to the location of the most proximal lymph node, also called the sentinel node. This sentinel node is excised and evaluated. Blunt dissection using vital dyes for guidance requires considerable surgical experience and talent associated with the delicate task of following the blue duct (a ruptured dye-carrying duct can be problematic). The ability to identify a tumor-free sentinel lymph node will enable the surgeon to accurately stage metastasis-free breast cancer patients without subjecting them to the risks of radical dissection. The approach may also improve histologic staging by enabling the pathologist to focus on fewer lymph nodes.
Lymph node involvement in metastasis also has been the subject of investigation in other quite different forms of cancer such as colon cancer. This has been through the utilization of a hand-held radiation responsive probe. The U.S. Pat. No. 4,782,840 by Martin et al. entitled "Method for Locating, Differentiating, and Removing Neoplasms", issued Nov. 8, 1988, reviews the approaches of nuclear medicine for locating colon tumors. The patent discloses a method for locating, differentiating, and removing neoplasms by using a radio-labeled antibody in conjunction with the radiation detection probe, which the surgeon may use intraoperatively in order to detect the sites of radioactivity. Because of the proximity of the detection probe to the labeled antibody, the faint radiation emanating from occult sites becomes detectable because, in part, of the inherent application of the approximate inverse square law of radiation propagation. This evaluation also may be employed with certain more minimally invasive procedures as described by M. W. Arnold, and M. O. Thurston, in U.S. Pat. No. 5,383,456, entitled "Radiation-Based Laparoscopic Method for Determining Treatment Modality" issued Jan. 24, 1995.
Thurston et al. discloses a radiation based method for locating and differentiating sentinel nodes in U.S. Pat. No. 5,732,704. The method identifies a sentinel lymph node located within a grouping of regional nodes at a lymph drainage basin associated with neoplastic tissue. A radiopharmaceutical is injected at the situs of the neoplastic tissue. The radiopharmaceutical migrates along a lymph duct toward the drainage basin containing the sentinel node. A hand-held probe with a forwardly disposed radiation detector crystal is maneuvered along the duct while the clinician observes a graphical readout of count rate amplitudes to determine when the probe is aligned with the duct. The region containing the sentinel node is identified when the count rate at the probe substantially increases. Following incision, the probe is maneuvered utilizing a sound output in connection with actuation of the probe to establish increasing count rate thresholds, followed by incremental movements, until the threshold is not reached and no sound cue is given the surgeon. At this point of the maneuvering of the probe, its detector will be adjacent to the sentinel node, which then may be removed. Although this procedure is currently possible, the random nature of decay of the radiopharmaceutical, and the low doses desired to minimize patient and clinician exposure, provide less than optimal guidance of current devices such as those described in U.S. Pat. No. 5,732,704.
An attempt to compensate for the difficulty in determining count rates has been described in U.S. Pat. No. 4,889,991. An enhanced signal treatment algorithm was developed utilizing weighted averaging and slew rate limiting to provide enhanced audio output for cueing an operator about a radiation detector probe's position relative to a radiolabeled sentinel node. It describes the use of a squelch determined by establishing a base count rate for background radiation. Where a statistically significant count rate is encountered, and depending upon the system's operational mode, the presence of a tumor will be defined as, for example, a count rate of three standard deviations above the base count rate. However systems utilizing signal processing algorithms such as those described in U.S. Pat. No. 4,889,991 lose some precision and response time due to the squelching and averaging of count rate information. Count rate averaging and weighted averaging can be characterized as linear filters, and specifically as low-pass filters. Low pass filters tend to slow system response time and reduce the strength of the response of the system.
Therefore, it would be advantageous to provide a radiation detection and guidance method that has an improved response rate to precisely locate sentinel nodes. It would be advantageous to provide a count rate determination method that is less hindered by the tradeoff between system stability and system response time inherent in current linear algorithms. It would also be advantageous to provide a radiation detection and guidance method that provides conditioned output count rate information to reduce erroneous or spurious changes in feedback to the operator. It would further be advantageous to provide improved count rate information to actual changes of count rates while removing rate change artifacts due to the random nature of radiation decay.